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FUKUSHIMA, Japan — They risk explosions, fire, and an invisible enemy — radiation that could kill quickly or decades later — as they race to avert disaster inside a dark, overheated nuclear plant.

The 180 emergency workers at Japan’s crippled Fukushima Daiichi complex are emerging as heroes in the midst of a disaster spawned by an earthquake and a tsunami. Dubbed by some as modern-day samurai, the technicians were ordered back to work late yesterday after a surge of radiation forced them to leave their posts for hours.

“I don’t know any other way to say it, but this is like suicide fighters in a war,’’ said Keiichi Nakagawa, associate professor of the Department of Radiology at the University of Tokyo Hospital.

Small teams of the still-anonymous emergency workers rush in and out for 10 to 15 minutes at a time to pump seawater into the plant’s overheated reactors, monitor them, and clear debris from explosions. Any longer would make their exposure to radioactivity too great.

Even at normal times, workers wear coveralls, full-face masks with filters, helmets, and double-layer gloves when they enter areas with a possibility of radiation exposure.

But the burst of radioactivity yesterday led the government to order an evacuation of the Daiichi complex. “The workers cannot carry out even minimal work at the plant now,’’ Chief Cabinet Secretary Yukio Edano said of the temporary pull out.

The highest reading among various locations that had to be accessed by the workers hit 600 millisieverts, equal to several years of daily exposure limit, according to statistics released by Tokyo Electric Power Co.

Millisieverts measure exposure to radiation, which can cause cancer and birth defects. Severe exposure can cause burns and radiation sickness — nausea and vomiting and harm to blood cells.

A typical individual might absorb 6 millisieverts a year from natural and man-made sources such as X-rays. Small additional annual exposures of under 100 millisieverts are believed to produce no discernible harm, but more carries health risks.

Tony Irwin, an Australia-based nuclear consultant, said the normal dose for a radiation worker is 20 millisieverts a year, averaged over five years, with a maximum of 50 millisieverts in any one year.

“So they would be trying to rotate people to make sure they’re within that limit. Now many countries have an emergency limit of 100 msvs a year,’’ he said. “They’ll wear radiation monitors, so they can see exactly what they’re getting on a real time basis.’’

Yet, Japan’s Ministry of Health Labor and Welfare raised the maximum legal exposure for nuclear workers to 250 millisieverts from 100 millisieverts. It described the move as “unavoidable due to the circumstances.’’

The workers’ challenges this week have included struggling for hours to open a pressure-release valve and allow water to enter the reactors. When a worker left the scene for a short period, the water flow ceased and fuel for pumps bringing up the water ran out.

A building housing a spent fuel storage pool exploded at one point, making two huge holes on the upper side of the wall on the building. A plant worker spotted a fire shortly thereafter that was later put out.

The workers also have had to walk around the area to measure radioactivity in each place they were supposed to enter and remove contaminated debris. They also struggle with broken equipment, often working in darkness.

“The thing I’ve been concerned about right now are the workers. They are at a tremendous risk,’’ said Don Milton, a doctor who directs the institute of applied environmental health at the University of Maryland.

Milton noted reports that some workers have already shown signs of acute radiation sickness. That would be even worse than it sounds because “the sooner it comes on after exposure, the worse it is.’’